PECTORAL. SKELETON OF TELEOSTEANS. 369 
that the preecoracoid foramen is a considerable distance from 
its anterior border. 
The mesocoracoid (fig. 6, m.) is now fully developed, having 
extended up to and united with the top of the scapula. It is 
a significant fact that the mesocoracoid remains dormant so 
long and then develops so rapidly. The rapidity of its 
development may be judged from the fact that the young in 
my Stages I—III vary in length from 15°5 mm. to 22 mm., 
whilst those of Stage IV are only 25 mm. long. This is no 
doubt associated with the equally rapid rotation of the 
coracoid into the horizontal position; and just as the scapula 
rises to form a buttress for the outer end of the glenoid 
border, so the mesocoracoid does likewise for the inner end. 
In this connection it is interesting, as further evidence, to 
compare the position of the base of the mesocoracoid as seen 
in figs. 5, 6, and 8, and to note its increasing proximity to 
this border. Ducret observed the late appearance of the 
mesocoracoid (p. 21). 
The growth of the precoracoid process has been almost 
stationary, and so in consequence of the forward growth of 
the scapulo-coracoid region it has been pulled away from that 
intimate relation to the cleithrum which it has had up to 
this stage. Nevertheless it now meets its fellow in the 
middle line. 
The postcoracoid process is still present, but is greatly 
reduced. 
The Adult (fig. 8).—Already in the salmon 52 mm. long, 
the girdle is as fully ossified as in the adult, and unfortu- 
nately I have no stages between Stage IV and this. Con- 
cerning the adult little need be said here, for it has been 
well described by Parker, though he overlooked the pre- 
coracoid foramen. 
The Development of the Pectoral Skeleton in 
Gasterosteus aculeatus. 
Stage I (fig. 9)—Comparing this with the corresponding 
stages of the salmon one is struck at one time by the great 
